Developments in zeolite catalysts and hydrocarbon conversion processes have created interest in utilizing light aliphatic feedstocks for producing C.sub.5 + gasoline, diesel fuel, etc. In addition to basic chemical reactions promoted by medium-pore zeolite catalysts, a number of discoveries have contributed to the development of new industrial processes. These are safe, environmentally acceptable processes for utilizing aliphatic feedstocks. Conversions of C.sub.2 -C.sub.4 alkenes and alkanes to produce aromatics-rich liquid hydrocarbon products were found by Cattanach (U.S. Pat. No. 3,760,024) and Yan et al (U.S. Pat. No. 3,845,150) to be effective processes using the zeolite catalysts. In U.S. Pat. Nos. 3,960,978 and 4,021,502, Plank, Rosinski and Givens disclose conversion of C.sub.2 -C.sub.5 olefins, alone or in admixture with paraffinic components, into higher hydrocarbons over crystalline zeolites having controlled acidity. Garwood et al have also contributed to the understanding of catalytic olefin upgrading techniques and improved processes as in U.S. Pat. Nos. 4,150,062, 4,211,640 and 4,227,992. The above-identified disclosures are incorporated herein by reference.
Catalytic dehydrogenation and aromatization of light paraffinic streams, e.g. C.sub.2 -C.sub.4 paraffins, commonly referred to as LPG, is strongly endothermic and typically carried out at temperatures between 540.degree. and 820.degree. C. (1000.degree. and 1500.degree. F.). While the incorporation of hydrogenation/dehydrogenation metals including gallium, platinum, indium, tin and mixtures thereof in zeolite catalysts may reduce the operating temperature to the range of about 400.degree. to 600.degree. C. (750.degree. to 1100.degree. F.), the problem of transferring sufficient heat to a catalytic reaction zone to carry out the paraffin upgrading reaction remains as an obstacle to commercialization of these processes.
Dehydrogenation of paraffins to olefins has recently generated increasing interest as the market value of olefinic intermediate feedstocks continues to rise. Light olefins, particularly C.sub.2 -C.sub.4 olefins enjoy strong demand as building blocks for a wide range of valuable end products including fuels and specialized lubricants as well as thermoplastics.
Methods of supplying heat to an endothermic reaction zone also include indirect heat exchange as well as direct heat exchange. Indirect heat exchange is exemplified by a multi-bed reactor with inter-bed heating or a fluid bed reactor with heat exchange coils positioned within the catalyst bed. Direct heat exchange techniques include circulation of inert or catalytically active particles from a high temperature heat source to the reaction zone, or the coupling of a secondary exothermic reaction with the primary endothermic reaction in a single catalytic reaction zone. An example of such a secondary exothermic reaction is the oxidative dehydrogenation of a portion of the feedstream.
Known techniques for oxidative dehydrogenation are unfortunately less than 100% selective and at least a part of the valuable product is oxidized, adversely affecting not only selectivity and yield but also accelerating permanent steam deactivation of the zeolite catalyst by exposing the catalyst to the water of combustion at elevated reaction temperatures. Further, the incremental costs associated with maintaining a controlled supply of a suitable oxygen source, e.g. NO.sub.x, CO.sub.2 or SO.sub.3, detracts from the commercial potential of such techniques.
Examples of such oxidative dehydrogenation processes include U.S. Pat. No. 3,136,713 to Miale et al teaches a method for heating a reaction zone by selectively burning a portion of a combustible feedstream in a reaction zone. Heat is directly transferred from the exothermic oxidation reaction to supply the endothermic heat for the desired conversion reaction.
Heat balanced reactions are also taught in U.S. Pat. No. 3,254,023 and 3,267,023 to Miale et al. Additionally, U.S. Pat. No. 3,845,150 to Yan and Zahner teaches a heat balanced process for the aromatization of hydrocarbon streams by combining the exothermic aromatization of light olefins with the endothermic aromatization of saturated hydrocarbons in the presence of a medium-pore zeolite catalyst.
Turning now to chemical reaction kinetics, it is well recognized that the extent of reaction may be increased by removing reaction products from contact with the reactants as the reaction products are formed. This principle finds application in U.S. Pat. No. 3,450,500 to Setzer et al. which teaches a process for reforming hydrocarbon feedstocks and withdrawing the hydrogen product from contact with the feedstock driving the equilibrium to favor increased hydrogen production.
Similarly, British Patent Application GB 2190397 A describes a process for producing aromatic hydrocarbons by catalytic paraffin dehydrocyclodimerization. The process upgrades C.sub.2 -C.sub.6 paraffins, i.e. ethane, propane, butane or a mixture thereof to a mixture of aromatic hydrocarbons and hydrogen by-product in a reactor provided with a membrane capable of selective, in-situ transfer of at least a portion of the hydrogen in the mixture across the membrane. Catalysts useful in the paraffin upgrading process are said to include zeolites, and in particular gallium-containing zeolites.
It is believed that the paraffin dehydrogenation reaction is equilibrium limited when carried out in a conventional reactor due to relatively high hydrogen partial pressure Thus the state of the art of endothermic hydrogen-producing paraffin upgrading processes would clearly be advanced by a process and apparatus for increasing the extent of reaction while also providing a high temperature heat source to supply at least a portion of the endothermic heat of reaction.